This invention relates to a continuously variable transmission system for rear wheel drive vehicles and more particularly to a compact arrangement for such a transmission where the continuously variable ratio assembly is offset relative to the engine crankshaft center line. In one embodiment, the input shaft is in-line with the output shaft. In another embodiment, the input shaft is out-of-line with the output shaft.
Continuously variable transmissions generally utilize a pair of adjustable pulleys, including a primary pulley and a secondary pulley. The primary pulley is connected to an engine and the secondary pulley is connected to the drive train of the vehicle, typically through a clutch. One example of such a continuously variable transmission ("CVT") is shown in Smirl, U.S. Pat. No. 4,433,594 entitled "Variable Pulley Transmission" and incorporated herein by reference.
Generally, a drive belt interconnects the pulleys. Specifically, the drive belt transfers power from the primary pulley to the secondary pulley, i.e., from the engine to the drive train, by means of frictional contact between the drive belt and the pulleys.
Each pulley is constructed from two flanges and each flange has a conical side surface to define a generally V-shaped gap between the flanges. At least one of the flanges is movable along the axis of the shaft to allow the gap between the flanges to be varied. In such a fashion the transmission ratio of the CVT can be varied, i.e., changing the effective gap width between the flanges of the two pulleys varies the radial position of the drive belt in each pulley. This allows for continuous adjustment of the drive ratio between the shafts and thus between the engine and the drive train.
Movement of the flanges is achieved generally through a hydraulic servo. Specifically, the servo has a fluid constraining chamber. Increasing fluid pressure in this chamber causes the flange to move axially and thus increases the effective diameter of the pulley As fluid pressure is decreased in this chamber, the flange moves along the shaft in the opposite direction due to the tension on the belt, thus decreasing the effective diameter of the pulley. Generally, the effective diameter of the primary pulley is moved in one direction as the effective diameter of the secondary pulley is moved in the opposite direction. A further and more detailed description of servo operation and flange movement may be found in Neuman et al. U.S. Pat. No. 5,006,092 entitled "Continuously Variable Transmission Power Train Configuration" and incorporated herein by reference. Various servo and flange configurations may be used to provide an adjustable pulley. Any suitable design known to those skilled in the art may be used.
In an automobile installation space constraints limit the volume into which a transmission can be installed and thus limit the availability of using a CVT. Because a typical CVT utilizes a pulley assembly constructed from two pulleys positioned on parallel shafts and linked with a belt, the minimum width required to install a CVT is determined by the size of the belt and pulley assembly. Rear wheel drive automobiles, however, offer only limited space in the transmission tunnel for the installation of a transmission, especially because the transmission tunnel in such automobiles is underneath the forward section of the passenger compartment. Typically, this space provides greater length than height or width. In the past, therefore, passenger space considerations have limited the use of a CVT system.
The present invention solves this problem by providing a compact CVT system. One embodiment of the present invention provides a compact arrangement for a CVT where the input and output shafts are coaxial to allow installation into a rear wheel drive automobile without modification to the transmission tunnel or without requiring additional gears or modified differentials on the drive train.
Another embodiment of the present invention provides a compact arrangement for a CVT where the variable ratio assembly is offset relative to the input shaft and the output shaft is in line with the secondary pulley of the variable ratio assembly.